RDs for 20" wheel trikes

[UpWrong]

I'm planning on changing my ICE Sprint to a 20" rear section and pondering what RD to fit. Normally the bigger the capacity the better but with a 20" wheel the risk of knocking and bending the RD increases. And the cage length of Shimano RDs has gotten longer to accommodate larger sprockets.

Shimano brand their RDs as SS, GS and SGS for short, medium and long cages. For a triple crankset, you usually want an SGS cage but I notice the Sora RD-R3000 is branded GS with a capacity of 43T. This sounds similar to the long cage SRAM X series which was a staple install on 20" wheel ICE trikes. Their cages were not as long as Deore SGS.

However, I have a box full of RDs so will investigate their cage lengths and report back. My Sprint has a bar end shifter so not compatible with the SRAM X RDs unless I employ friction mode. With friction mode, I wouldn't be restricted to 9-speed but could contemplate using 10 or 11 speed cassettes with closer ratios. That would need new chains but there's an opportunity to save weight there.

[UpWrong]

I have a Claris RD R2000 which has the same form as the Sora R3000, GS label and a capacity of 43T. It's nominally 8-speed but I can't see why it wouldn't function with a 9-speed chain and cassette.

[UpWrong]

I've had a quick rummage through my box of RDs and can confirm the Claris GS cage is a little shorter than an SGS Deore cage and about the same as that of a long cage SRAM X5, so the Claris or Sora GS is a good option for a 20" wheel.

[Tigerbiten]

In the 2 years I used a rear derailleur with a 20" back wheel, I think I hit the derailleur once on the ground.
Slipped sideways off a rock while offroad and the derailleur hit the rock.
Otherwise I found it almost impossible with normal riding as the back wheel lifted the derailleur above everything.

What was more normal was filling the derailleur with grass when crossing a field when offroad.
Then having to dig the oily mess out of it after the field ended due to increased drag.

[UpWrong]

In the 2 years I used a rear derailleur with a 20" back wheel, I think I hit the derailleur once on the ground.
Slipped sideways off a rock while offroad and the derailleur hit the rock.
Otherwise I found it almost impossible with normal riding as the back wheel lifted the derailleur above everything.

What was more normal was filling the derailleur with grass when crossing a field when offroad.
Then having to dig the oily mess out of it after the field ended due to increased drag.

Interesting. I have seen RDs that extend below the wheel rim, so would seem to be vulnerable in the event of a puncture.

Shadow RDs sit closer to the wheel so might have less risk of being bashed by a passing rock, but many don't have cable adjusters.

[Tigerbiten]

I had a long cage X9 and used it's maximum range, 48-22 in front and 9-32 on the back, 12.5"-100".
Set it up so large-large worked but near the small-small combo the chain started to rub on the derailleur cage which was fine as I didn't need those duplicated combo's.

[Tigerbiten]

If I was setting up a 20" wheel derailleur setup now, I'd probably go with a SA 3 speed CS-RF3 with a 9 speed 11-28 cassette and a 24-36-48 triple up front, 12"-108" range. A 9x total gear range with close spaced gears at the top end.
The only reason I didn't go with a similar setup on the major upgrade of the trike is that with only 1 hand, I'd end up with too many controls on the one side.

[UpWrong]

I shall probably go with the Claris R2000 RD (weight 275g) with an 11-32 SLX HG80 cassette (284g) and a 28-38-50 triple up front, so using the full 43T capacity. Rear tyre 49-406 Maxxis DTH 120 tpi (weight 350g). Gear range about 17"-90". Or possibly slightly lower with a 26-36-48 triple (16" to 86").

[Tigerbiten]

I'd push your front derailleur to it's limit. Measure the gap between your chain on the smallest chainring and the bottom of the cage. For every 2mm you can increase the difference between the large and small chainring by 1 tooth. So something like 26-38-50 should be possible or even 2T more if you really push the limits. That's how I ended up with a 22-36-48 triple.

The full 43T capacity of the rear derailleur is conservative, you can probably easily add 2-4 teeth to the capacity because as long as big-big works then it doesn't really matter if small-small is a little slack. You just remember not to use the smallest sprocket when on the small chainring as you can get the same gears with bigger sprockets on the big chainring. That's how I ended up with a 49T capacity with my derailleur gears.

I tend not to use "standard" setups. I now use a Schlumpf HSD with twin chainrings and a Rohloff. My next setup will hopefully be twin chainrings on a Shimano Steps drive and a Rohloff. I think it will work but may have "fun" setting it up.

Luck .....

[hercule]

If it’s any help I've a medium cage Shimano XT rear mech on my Moulton AM… that’s with 17” wheels. Works fine and no ground clearance issues. Largest sprocket is 32 teeth, smallest is 10. This is running a 9s transmission. The mech is at least 20 years old though, so not a current model!

[UpWrong]

My Metabikes came with a medium cage Tiagra RD , an 11-28 cassette and a 30-40-50 triple so it only needed 37T capacity. It's the large range cassettes that require longer cages.